scan: use full md filter when md 1.0 devices are present

scan: use full md filter when md 1.0 devices are present

The md filter can operate in two native modes:
- normal: reads only the start of each device
- full: reads both the start and end of each device

md 1.0 devices place the superblock at the end of the device,
so components of this version will only be identified and
excluded when lvm uses the full md filter.

Previously, the full md filter was only used in commands
that could write to the device.  Now, the full md filter
is also applied when there is an md 1.0 device present
on the system.  This means the 'pvs' command can avoid
displaying md 1.0 components (at the cost of doubling
the i/o to every device on the system.)

(The md filter can operate in a third mode, using udev,
but this is disabled by default because there have been
problems with reliability of the info returned from udev.)